Update the help message.

Update the help message displayed when the program is invoked as
'tcalc --help' to reflect the recently added features.
Add support for additional navigation keys.

Add support for the following navigation keys:
    Home key: pressing 'Home' moves the cursor to the first character of
    the currently visible expression string. If possible, the display is
    scrolled left one character to show the preceding character.

    Shifted Home key: pressing 'Shift-Home' moves the cursor to the very
    beginning of the expression string. If necessary, the display is
    scrolled left.

    End key: pressing 'End' moves the cursor past the last character
    currently visible on the display. If necessary, the display is
    scrolled right.

    Shifted End key: pressing 'Shift-End' moves the cursor to the very
    end of the expression string. If necessary the display is scrolled
    right.

Fix a bug in which a non-printable character is displayed if the length
of the expression string reached MAX_EXPR_LEN.

Update the help message according to the changes.